1. Field of the Invention
The present invention is directed to a hearing aid device of the type having different assemblies for the pick-up, further processing and for the adaptation of an acoustic signal to the hearing capability of a hearing-impaired person, the hearing aid device having an earphone for the acoustic output and an induction coil for inductive pick-up of signals and a compensation-inductance for the generation of a compensation field. Further, the invention is directed to a method for reducing the magnetic noise fields that emanates from the earphone given operation of a hearing aid device with an induction coil.
2. Description of the Prior Art
A hearing aid device of the general type described above is known from German PS 197 12 236. In this known hearing aid device, the harmonic distortion of the induction coil or telephone coil or hearing coil that occurs when the induction coil is operated is reduced or avoided. The stray fields, which trigger this harmonic distortion, are generated by the printed circuit board on which the induction coil is mounted. The stray fields act proportional to the load current changes and harmonic distortions therefore result.
Low-frequency magnetic fields of the environment can be picked up by means of the induction coil or telephone coil or hearing coil of the hearing aid device, these low-frequency magnetic fields being converted into signal voltages in the hearing aid device and amplified and subjected to a signal processing before they exit the earphone as acoustic signals. These components, for example, are arranged spaced from each other in the device housing and specific shielding plates are provided in order to shield the induction coil from the earphone. These known shielding plates, however, can influence only one component of the stray field, thus an optimally complete shielding of the earphone is not possible using this structure.
In a hearing aid device, in particular, of medium performance class and high performance class and given high or maximum amplifier adjustment, it has been found that magnetic couplings occur during the operation of the induction coil, these magnetic couplings partially leading to an oscillation of the device and thereby making the hearing aid ineffective. These couplings are triggered by parasitic stray fields of the earphone and have an effect on the induction coil.
An object of the present invention is to provide a hearing aid wherein magnetic couplings during operation of the induction coil are avoided or substantially reduced by an active shielding of the earphone.
The above object is achieved in accordance with the principles of the present invention in a hearing aid having a pick-up stage for receiving an acoustic signal and converting the acoustic signal into an electrical signal, and a processing stage for processing the electrical signal and adapting it to the hearing disability of a hearing-impaired person, and an earphone to which the processed electrical signal is supplied for conversion into an output acoustic signal, and having an induction coil for inductively picking-up acoustic signals, and a method for operating such a hearing aid device, wherein a compensation inductance is provided for generating a compensation field, and wherein the compensation inductance is connected in the signal line between the processing stage and the earphone and wherein, during operation of the induction coil, the compensation inductance generates a compensation field which is directed toward the magnetic field produced by the induction coil and which opposes the magnetic field produced by the earphone, so as to actively shield the earphone and prevent the magnetic field produced by the earphone from influencing the induction coil, thereby producing unwanted noise signals in the inductively-generated current produced by the induction coil.
A fundamental wave compensation ensues in the invention; the signal amplitudes are influenced by the inductance and not the harmonics. Inventively, the compensation-inductance is an SMD coil permeated by the earphone current for achieving an active shielding of the earphone. The problem of magnetic couplings in the induction coil operation is simply solved by the utilization of an SMD coil. This inductance is switched into the earphone line, so the coil has the earphone current flowing therein, and the emitted compensation field corresponds to the parasitic stray field of the earphone. The compensation-inductance is directed toward the induction coil and the compensation field acts opposite to the earphone field at the same time, so that a negative feedback results, which leads to a suppression of the coupling. The strength of the negative feedback can be adjusted by the number of turns of the coil and/or by the arrangement of the compensation coil in the area of the induction coil. A specific advantage of the SMD coil is the ability to position it by machine equipping, so that the position of the SMD coil is always exactly definable during the manufacture of the hearing aid device. Furthermore, the manufacturing costs can be reduced, since shielding plates are no longer required. Moreover, the inventive hearing aid device can be constructed smaller, since the shielding plates are foregone and the earphone and the induction coil are arranged at a smaller distance from each other. Also, stray field tolerances of the earphone can be compensated by redimensioning the SMD coil.